1. Field of the Invention
The present invention relates to a method for aligning a light-receiving element array substrate provided in a contact image sensor in a sub-scanning direction, a method for manufacturing the contact image sensor, and the contact image sensor.
2. Related Background Art
A contact image sensor, which enables reading the image of an original, comprises a light source, a lens, a light-receiving element array (e.g. CCD) and a housing for containing them. The original is irradiated by light from the light source, the lens focuses the light reflected from the original on the light-receiving element array, and the light-receiving element array receives the focused light. Although the light reflected from the original enters the lens and the light-receiving element in the contact image sensor receives the focused light by the lens, ghost images may be generated by the structure of an optical system causing a decrease in an image quality. Ghost images are a phenomenon such that a virtual image is generated in an image area in which an original image doesn't normally exist. The alignment of components, such as the light source, the lens, and the light-receiving element array, in the contact image sensor is important to prevent ghost images from being generated.
Conventionally, a rod lens array is used for the lens provided in the contact image sensor. An example of a rod lens array is disclosed in Japanese Patent Publication No. 1993-167778. Because the rod lens array is formed with a combination of a plurality of rod lenses disposed in an array configuration, each of the incident rays that enter the plurality of rod lenses generally has an optical path toward one of the plurality of rod lenses. Thus it is rare that the incident ray which enters into one rod lens enters into another adjacent rod lens. Therefore, in case that the rod lens array is used for the lens, ghost images are not typically generated. In addition, the rod lens array is generally formed by containing a light-shielding film between the rod lenses to obtain a more high-quality image, so that unnecessary light may not be entered into the adjacent rod lens.
As described above, the rod lens array has a structure such that ghost images are not typically generated. In case that the rod lens array is used for the contact image sensor, the precise alignment for the components, such as the rod lens array and the light-receiving element array, is typically not carried out.
However, a micro-lens array plate has more recently been used in contact image sensors instead of the rod lens array. On the micro-lens array plate, it is difficult to integrally form such a light-shielding film between micro-lenses, because the micro-lens array plate is formed integrally in the manufacturing process by means of injection molding of resin. In addition, the structure of the micro-lens array plate makes it easy to transmit unnecessary light when forming an image, since each of micro-lenses in the micro-lens array plate does not have a separate structure as compared with the rod lens array. Therefore, ghost images become a significant matter when utilizing the micro-lens array plate for the lens of a contact image sensor as opposed to utilizing the rod lens array for the lens of the contact image sensor where ghost images are not typically generated.
To prevent ghost images from being generated, it is generally disclosed, for example in Japanese Patent Publication No. 1999-331498 to form the light-shielding film into the micro-lens array plate in another additional process, and to form a slit opening in a slit plate located between the original and the micro-lens array plate. Even if the slit plate is used, however, ghost images may be generated when precise alignment among the slit plate, the lens and the light-receiving element array is not carried out. Therefore, it is especially important that each of the components, such as the slit plate, the lens and the light-receiving element array, are precisely aligned in a sub-scanning direction with respect to the optical system in the contact image sensor.
Additionally, a linear illuminating device is generally used for the light source provided in the contact image sensor. The linear illuminating device is typically composed of a plurality of light-emitting elements, such as light-emitting diodes or light-emitting thyristors which are linearly mounted on a light-emitting element array substrate that is composed of one or more of the light-emitting elements designed on one end of a translucent linear light-guide, for example. In such a linear illuminating device, it is necessary to align a positioning relationship between the slit plate and the lens to prevent ghost images from being generated, and to align a positioning relationship between the slit plate and the lens for increasing sensor-output, which corresponds to increasing of amount of light. The sensor-output, as described herein indicates a value of an output voltage of the contact image sensor, the output voltage being a value from a photo-electric converted signal by means of the light-receiving element array. An example of technology for aligning an array-line of the light-receiving element array such that a positioning relationship between a slit opening in a slit plate and an array-line of the light-receiving element array becomes parallel is disclosed in Japanese Patent Publication No. 1993-122443.
When the micro-lens array plate is used for the contact image sensor, as described above, the precise alignment of the micro-lens array plate, the slit plate and the light-receiving element array is needed. In order to carry out a precise alignment, the presence or absence of ghost images is desirably confirmed. A generalized method of checking for the presence or absence of ghost images is to read a whole area of a test chart 12 including round patterns 10 (as shown in FIG. 1A) through the optical system of the contact image sensor, and then to read the output image of the sensor-output. In this case, if the positioning relationship among the micro-lens array plate, the slit plate and the light-receiving element array is misaligned, ghost images 16 may appear as output image 14, as shown in FIG. 1B. In contrast, output image 18, shown in FIG. 1B is an actual image of the round patterns 10.
However, when the test chart 12 shown in FIG. 1A is used, there is a problem of taking more time for the alignment because the whole area of the test chart 12 should be read, and of a difficulty of online alignment.
Moreover, the light source provided in the contact image sensor generally has an irregular distribution of amount of light. In addition, in the case of utilizing the micro-lens array plate for the optical system, there is another problem in that the micro-lens array plate, especially, increases the irregular distribution of amount of light corresponding to the sensor-output relative to the case of utilizing another lens (e.g. a rod lens array). It is typically difficult to form the micro-lens array plate while keeping a uniformity of the dimensions (in particular, the plate thickness) and an optical performance of micro-lenses (a direction of an optical axis, a lens pitch and a curvature of the lens) on the whole area of the micro-lens array plate in the injection molding process.